In the advanced lithography process for the fabrication of semiconductor devices, a light source of shorter wavelength is used for exposure. A subsequent transition to lithography using extreme ultraviolet (EUV) is regarded promising.
In the EUV lithography using EUV light with a short wavelength of 13.5 nm, a reflecting optical system is employed because no materials having a high transmittance at such short wavelength are available. Reflection of EUV light is undertaken by a Si/Mo multilayer coating sputtered on a low thermal expansion material substrate.
One of the most serious problems which must be overcome before EUV lithography can be implemented in practice is the manufacture of defect-free photomasks. Although defects like irregularities on the surface of photomask substrate are permissible in the conventional KrF lithography (wavelength 248.3 nm) and ArF lithography (wavelength 193.4 nm) relying on the refractive optical system, defects of the same order are not negligible in the EUV lithography because of the short wavelength and the reflecting optical system used therein.
Further, EUV lithography members, especially photomask substrates are required to be fully flat. At the practical level, photomask substrates must have a very high flatness of up to 30 nm within a central region of 142 mm×142 mm squares.
Low thermal expansion materials known useful as EUV lithography members include titania-doped quartz glass. However, it is difficult to manufacture substrates having a high flatness from titania-doped quartz glass when the glass has a non-uniform titania concentration. When a glass substrate having a non-uniform titania concentration is polished, the substrate surface becomes irregular due to varying reactivity with the polishing slurry and differential grinding speed. In this regard, Patent Document 1, for example, discloses that titania-doped quartz glass having a narrow titania concentration distribution is useful as EUV lithography members.
Patent Document 2 refers to the refractive index distribution of titania-doped quartz glass that is determined by taking into account the polishing mechanism so that high-flatness substrates may be readily manufactured therefrom.
During manufacture of titania-doped quartz glass, zones having a non-uniform titania concentration, known as striae, may be formed perpendicular to the growth direction of titania-doped quartz glass, due to temperature variations at the growth face, variations of the reactant gas composition, and other factors. Striae are generally variations of titania concentration at intervals of several microns to several millimeters, and structurally strained sites are present within the stria. Since strained sites within titania-doped quartz glass are structurally unstable, selective abrasion occurs thereat during polishing, leading to aggravated flatness. Based on the discovery that striae-strained sites are converted into numerical values of stress, Patent Document 3 discloses the stress level permissible as EUV lithography members and the method of reducing the stress.
Patent Document 4 discloses that a plate member in which striae planes are parallel to the plate surface is used to prevent striae from being exposed and minimize the impact thereof.
Patent Document 5 discloses a method of removing striae from titania-doped quartz glass by using a zone melting method to apply shear stresses to the glass.